As the coatings industry, especially the automotive coatings industry, attempts to move to the use of coating compositions having a high content of essentially nonvolatile solids (commonly called high-solids coating compositions) the problems of providing coating compositions having a high solids content which still provide desirable performance characteristics have become increasingly challenging.
One problem arises from the tendency of high-solids thermosetting compositions to sag on a substantially nonhorizontal substrate during the heat curing operation. Such sagging adversely affects the appearance properties of both pigmented and unpigmented cured films. However, the adverse effects of sagging on appearance properties of cured films are particularly severe when the high-solids coating compositions contain metallic-flake pigments. Additionally, whereas some conventional additives for rheology control may provide some measure of sag control, other appearance problems of the cured films to a large degree are not improved by the addition of conventional rheology modifiers. For example, control of pigment orientation in metallic high-solids coatings so as to provide an acceptable "brightness of face" or "lightness of face" in the cured films along with a desirable two tone appearance of the coated substrate as it is viewed from different angles to a direction normal to the surface is particularly difficult where high-solids coating compositions are utilized. As used herein, control of the aforementioned two appearance properties collectively will be referred to as "pattern control". Moreover, some materials such as pyrogenic silicas which have been advocated for alleviating sag control problems in high-solids coating applications, while providing a measure of sag control, do not allow for both an adequate degree of pattern control and an adequate degree of gloss of the cured films prepared from coating compositions containing pyrogenic silicas. That is, when employed at a level which will provide a reasonable degree of pattern control, pyrogenic silicas tend to greatly lower the degree of gloss of the cured film. This performance disadvantage of materials like pyrogenic silicas is especially important considering the high gloss requirements for coating compositions for use in automotive applications.
It has now been found that alleviation of the above problems can be obtained by the incorporation in a high solids coating composition of substantially colorless, substantially inorganic microparticles ranging in size from about 1 to about 150 nanometers in the form of a sol of the microparticles in which the surface of the inorganic microparticles is essentially free of carbon-containing molecules chemically bonded to the inorganic microparticles. Applicants' patent application titled HIGH-SOLIDS COATING COMPOSITION FOR IMPROVED RHEOLOGY CONTROL CONTAINING CHEMICALLY MODIFIED INORGANIC MICROPARTICLES, filed even date herewith, is directed to a high-solids coating composition containing substantially colorless, substantially inorganic microparticles ranging in size from about 1 to about 150 nanometers in the form of a sol of the microparticles in which carbon-containing molecules are chemically bonded to the surface of the inorganic microparticles.